Chapter 17: Selection And Evolution

Question 1

What is genetic variation caused by?

Answer 1

■ independent assortment of chromosomes, and
therefore alleles, during meiosis
■ crossing over between chromatids of homologous
chromosomes during meiosis
■ random mating between organisms within a species
■ random fertilisation of gametes
■ mutation.

Question 2

What is the difference between mutation and the other factors that cause genetic variation?

Answer 2

Mutation can produce completely new alleles. The other factors that cause genetic variation allow for the reshuffling of existing alleles in the population.

Question 3

What produces phenotypic variation?

Answer 3

Offspring have combinations of alleles which differ from those of their parents and from each other. This genetic variation produces phenotypic variation.

Question 4

Is variation caused by the environment passed on to the offspring?

Answer 4

No

Question 5

What are some characteristics of discontinuous(qualitative) variation?

Answer 5

• A qualitative difference that has clear distinguishable
categories with no intermediates
■ different alleles at a single gene locus have large effects on the phenotype
■ different genes have quite different effects on the
phenotype.

Question 6

What are some characteristics of continuous(quantitative) variation?

Answer 6

• A quantitative difference that has a wide range of
phenotypes
■ different alleles at a single gene locus have small effects on the phenotype
■ different genes have the same, often additive, effect on the phenotype
■ a large number of genes may have a combined effect on a particular phenotypic trait; these genes are known as polygenes.

Question 7

What is fitness?

Answer 7

Fitness is the capacity of an organism to survive and transmit its genotype to its offspring. It is the extent to which organisms are adapted to their environment.

Question 8

What do selection pressures increase?

Answer 8

Selection pressures increase the chances of some alleles being passed on to the next generation, and decrease the chances of others, causing changes in allele frequency.

Question 9

What is natural selection and what does it do?

Answer 9

Natural selection is the effect of selection pressures on the frequency of alleles in a population. Natural selection raises advantageous allele frequencies and reduces the disadvantages allele frequencies within the population.
Natural selection occurs to limit exponential growth of populations and ensure survival of the fittest.

Question 10

What are the two types of environmental factors that affect population growth?

Answer 10

o Biotic – caused by living organisms e.g. predation,
competition for food, infection etc.
o Abiotic – caused by non-living organisms e.g. water
supply, nutrient levels in soil etc.

Question 11

What are the three types of selection pressures?

Answer 11

Stabilising, directional and disruptive selection

Question 12

What is stabilising selection?

Answer 12

Stabilising selection: when natural selection keeps the
variety of the population the same.
o If wide variation shown, selection pressure acts
against the two extremes.
o Results in a population with a narrower range of the
characteristic
o Tends to keep the variation in a characteristic centred around the same mean value

Question 13

What is directional selection?

Answer 13

Directional selection: when a new environmental factor or a new allele appears, causing different allele
frequencies to be produced
o Selection acts against one extreme, results in change
in a characteristic in a particular direction

Question 14

What is disruptive selection?

Answer 14

Disruptive selection: when conditions favour both
extremes of a population (this selection maintains
different phenotypes)

Question 15

Describe what the two selection pressures are on the sickle cell alleles.

Answer 15

■ Selection against people who are homozygous for the sickle cell allele, HbSHbS, is very strong, because they become seriously anaemic.
■ Selection against people who are homozygous
HbAHbA, is also very strong, because they are more
likely to die from malaria.

Question 16

Why do heterozygotes with the sickle cell trait have a strong selective advantage?

Answer 16

They do not suffer from sickle cell anaemia and are much less likely to suffer badly from malaria.

Question 17

What is genetic drift?

Answer 17

Genetic drift is a change in allele frequency due to

chance and is most noticeable in small populations

Question 18

What is the founder effect?

Answer 18

Further genetic drift of the small population leads to
further alteration of the allele frequency, leading to
evolution and a new species – Founder’s effect

Question 19

What is the Hardy-Weinberg principle used to find?

Answer 19

Used to calculate allele, genotype and phenotype

frequencies within a large randomly mating population

Question 20

When does Hardy-Weinberg principle not apply?

Answer 20

o Significant selective pressure against a genotype
o Migration into or out of population
o Non-random mating
o Limited population

Question 21

What is artificial selection?

Answer 21

When humans purposefully apply selection pressures to populations, the process is known as artificial selection.

Question 22

What factors are used in selective breeding in relation to cattle?

Answer 22

1. Progeny testing is carried out- A bull cannot
   be assessed for milk production since this a sex-limited
   trait. Instead, the performance of the bull’s female offspring is looked at to see whether or not to use the bull in further crosses.
2. Background genes- Within each organism’s genotype are all the alleles of genes that adapt it to its particular environment. These genes are called background genes. If chosen parents come from the same environment and are from varieties that have already undergone some artificial selection, it is likely that such parents share a large number of alleles of background genes, so the offspring will be adapted for the same environment.
3. Increased milk trait

Question 23

How is selective breeding used to improve disease resilience in plants?

Answer 23

Organisations are set up to screen seed collections for plants with traits such as disease resilience or climate resilience or efficient use of nitrogen fertilisers.

Question 24

Why do wheat plants now have shorter stems and how?

Answer 24

This makes them easier to harvest and means they have higher yields (because they put more energy into making seeds rather than growing tall). The shorter stems also make the plants less susceptible to being knocked flat by heavy rains, and means they produce less straw, which has little value and costs money to dispose of. Most of the dwarf varieties of wheat carry mutant alleles of two reduced height (Rht) genes. These genes code for DELLA proteins which reduce the effect of gibberellins on growth. The mutant alleles cause dwarfism by producing more of, or more active forms of, these transcription inhibitors. A mutant allele of a different gene, called ‘Tom Thumb’, has its dwarfing effect because the plant cells do not have receptors for gibberellins and so cannot respond to the hormone.

Question 25

What is inbreeding depression?

Answer 25

Inbreeding plants causes gene pool to become progressively smaller and weaker over generations

Question 26

What does out breeding achieve and how?

Answer 26

Out breeding produces heterozygous plants that are
healthier and grow taller.
▪ The aim is to get heterozygous and uniformity in
genes thus out breeding cannot be used
▪ Therefore, hybrid plants are used instead.
▪ Seeds are taken from companies which inbreed to
produce homozygous plants. These are crossed with
different homozygous varieties to produce different
hybrid offspring.

Question 27

What two factors in selective breeding increases crop improvement?

Answer 27

o Introduction of disease resistance as there is a great loss of yield resulting from infections
o Incorporation of mutant alleles: These genes code for DELLA proteins that reduce the effect of gibberellin
and produce shorter stems. This allows more energy to
be used for producing more seeds/grains instead of
growing tall.

Question 28

why does evolution occur?

Answer 28

Evolution occurs because natural selection give some alleles a better chance of survival over others.

Question 29

What are the observations and deductions of the theory of evolution?

Answer 29

Observation
• Organisms produce more offspring than what is needed to replace the parents (reproductive potential)
• Natural populations tend to remain stable in size
Deduction
• There is a competition for survival
Observation
• Variation amongst individuals of a given species
Deduction
• The best adapted variants will be selected by natural
conditions, these are the variants that have a selective
advantage and so ‘survival of the fittest’ occurs

Question 30

What is speciation?

Answer 30

Speciation is the evolutionary process by which new

biological species arise.

Question 31

What is reproductive isolation?

Answer 31

It is the inability of a species to breed successfully with related species.

Question 32

How does prezygotic reproductive isolation take place?

Answer 32

`o Individuals don’t recognise each other as mates or
don’t respond to mating behaviour
o Physically being unable to mate
o Inability to fuse male and female gametes
o Incompatibility of pollen and stigma in plants

Question 33

How does postzygotic reproductive isolation take place?

Answer 33

o Failure of cell division in the zygote
o Non-viable offspring (dies soon)
o Viable but sterile offspring

Question 34

Why are there different alleles geographically?

Answer 34

This is due to the selection pressures being different in different areas resulting in different alleles being selected for.

Question 35

What is allopatric speciation? *full

Answer 35

Allopatric speciation: when speciation occurs where two populations are separated from each other geographically
o Mixing of the two is prevented, and each have
different selection pressures acting on the populations
o This results in different alleles being selected for, and soon the morphological, physiological and behavioural
features become so different that the two populations
can no longer interbreed even if the barrier is removed

Question 36

What is sympatric speciation?

Answer 36

Sympatric speciation: is when a new species is evolved
from a species that inhabits the same geographic region
o The most common way in which sympatric speciation
occurs is by polyploidy
▪ A polyploidy is an organism with more than two
complete sets of chromosomes in its cell
▪ This can happen if meiosis is abnormal during gamete formation, resulting in a gamete with 2 sets of
chromosomes (diploid)
o If two diploid gametes fuse, it results in 4 complete
sets of chromosomes - tetraploid zygote
o If a diploid gamete joins with a haploid gamete they
form a triploid zygote
o Polyploidys are often sterile as they can’t divide during meiosis. This is because all the sets try to pair up at once and end up in a muddle.
o However, they can grow perfectly well & reproduce
asexually
o A tetraploid organism from a diploid
parent cannot interbreed successfully and become
different species

Question 37

What is an autopolyploid?

Answer 37

Autopolyploid: polyploidys that are derived from a

single species and are infertile.

Question 38

What is an allopolyploid?

Answer 38

Allopolyploid: polyploidys that are derived from
different species. Note that meiosis happens more
easily here as the sets of chromosomes are from
different species and can pair up. They are fertile, but
cannot breed with parent species (self-fertile)

Question 39

How is the amino acid sequence related to determining how closely related species are?

Answer 39

When the amino acid sequence of a particular
protein is compared in different species, the number of
differences gives a measure of how closely related the species are.

Question 40

Why is human mitochondrial DNA inherited through the female line?

Answer 40

Human mitochondrial DNA is inherited through the female line. A zygote contains the mitochondria of the ovum, but not of the sperm.

Question 41

How do changes in the nucleotide sequence in mtDNA occur?

Answer 41

Since the mitochondrial DNA is circular and

cannot undergo any form of crossing over, changes in the nucleotide sequence can only arise by mutation.

Question 42

What is the relationship between the differences in the nucleotide sequence of mtDNA and the length since they shared a common ancestor?

Answer 42

The greater the number of differences in the sequence of nucleotides, the longer ago those individuals shared a common ancestor

Question 43

What are some reasons why extinctions occur?

Answer 43

1. Change in climate
2. Increased competition from better adapted species.
3. Loss of habitats- Many species are adapted for survival in a particular habitat with a particular range of environmental conditions. There is a loss of habitats through draining wetlands, cutting down rainforests and polluting the air, water and soil.
4. Killing species for sport or food
5. A lack of political support for conservation
6. Increasing demand for certain products